Cytoplasmic Male Sterility

Summary

Modern commercial seeds are commonly grown using
plants with cytoplasmic male sterility (CMS).
Male sterile plants reduce the labor required to
make hybrids, but they can cause problems for
seed savers because the plants are male sterile in
all succeeding generations. Even heirlooms and
open pollinated seeds can be back-crossed onto
plants with CMS making them male sterile as well.

Discussion

Cytoplasmic Male Sterility is used extensively
by commercial seed companies as the simplest method of
protecting their seed lines. No lawyers or courts are
needed to prevent people from saving the seeds of a variety that
has sterility built into it by design.

I believe that the widespread use of CMS by commercial
seed companies is severely damaging the world's genetic
legacy.
CMS is inherited from the mother along with
the mitochondria and other organelles. In most
species, once CMS has been incorporated into a
variety it cannot be reversed. As an example:
I get queries from people about why
their carrots go wild when they try saving
carrot seeds. I believe it is due to CMS.
All hybrid carrots, and just about
all carrots from the grocery store
have CMS. So when a gardener plants carrot
roots hoping to get seed there may
not be any domestic carrot pollen
to fertilize the plants. The only available
pollen may be Queen Ann's Lace, which is
the wild ancestor of carrots. So the carrots
go wild in the first generation. For species without
a nearby wild ancestor or close relative,
no seeds will be produced.

Affected Species

Potatoes are so extensively damaged by CMS that
nearly every heirloom and every modern variety would need to be eliminated in order
to develop a healthy open pollinated population.

CMS is common in broccoli, cabbage, and radish. The brassicas are not
as heavily contaminated as other crops due to a self-incompatibility
mechanism which makes hybrids possible by other means than CMS.

Nearly all commercial carrots use CMS. Even open pollinated and heirloom
lines have been converted to CMS versions.

Open pollinated corn is somewhat contaminated with CMS due to the
widespread use of CMS in making hybrids prior to 1970 and the continuing use of
other CMS varieties since then.

Nearly all sugar beets use CMS. About half of red beets are contaminated with CMS.

Nearly all commercial sunflowers are CMS.

All hybrid onions are male sterile and many open pollinated
populations of onions are severely contaminated. The vast majority
of commercial onions are male sterile. Onion sterility is due to
an interaction between cytoplasmic DNA and nuclear DNA, so it can
be reversed with the use of proper pollen donors.

Spinach and the cucurbits are not severely affected due
to their monoecious or dioecious reproductive strategies. Peas and
Beans are such radical inbreeders that they are not commonly hybridized.
CMS is not widely available for tomato, but that is subject to change
in the near future.

What to do about it

I am actively eliminating male sterile plants from my farm by examining
each plant in my seed producing beds and weeding out any with CMS.

Potatoes: At the end of the 2011 growing season I made a potato
salad containing all potato tubers that were not abundantly fruitful.
In doing so I eliminated 95% of my potato genome. I am still saddened
by the loss, but I figure that long-term it was the best strategy.
In future years I will not knowingly save any potato seeds or tubers
unless they come from abundantly fruitful plants. Any potatoes that
come into my garden will be screened for fruitfulness before being
incorporated into my potato landrace.
Male fertile plants can be identified
by two methods: Tapping a male fertile flower releases a cloud of pollen.
Male fertile plants tend to be abundantly fruitful. One advantage of converting
to using only male fertile potatoes is that I will have volunteer potato seedlings in my
garden so I won't have to spend time and effort on
a formal potato breeding program. I can save
seeds and tubers from the best growing of the volunteer seedlings.

Carrots: Carrot flowers with CMS are easy to identify:
They don't have filaments or anthers, or the filaments/anthers are brown and shriveled up.
When seen from a distance a male fertile carrot flower looks fuzzy due to
so many filaments poking up. Male sterile carrot flowers look smoother.
Testing of my open
pollinated carrot landrace in 2011 showed that 70% of the plants were male sterile.
In the coming growing season the male sterile carrots will be eliminated from my
seed patch, and any new carrot varieties will be screened prior to incorporation
into my carrot landrace.

Male Fertile Carrot

Male Sterile Carrot

Onions, Allium cepa:
There are several forms of CMS in onion:
The flowers look normal, but there is no pollen on the anthers.
The flower heads may contain bulbils which is abnormal.
Drying seed-heads contain low percentage of pollinated flowers.
One form of
male sterility in onions is composed of a cytoplasmic factor and a recessive nuclear
gene. The sterility can be reversed by pollinating the male sterile plant with a
male-fertile plant. Then (some of) the F1 will shed pollen that can be used to pollinate
plants with normal cytoplasm. Since onion sterility is subject to Mendelian inheritance
as well as maternal inheritance they will undergo routine mass selection to
eliminate plants that don't shed pollen, and also to eliminate any flower heads
with low seed set or with bulbils.

Garlic: I don't know the mechanism causing sterility in garlic. Nevertheless
I'm including the work-around.
Remove bulbils from hard-necked garlic so that
flowers have a better chance of setting seed.
Specialized
techniques may allow soft-necked garlic to flower, and hard necked-garlic to set more viable seeds.

Corn: Tassels with type
T cytoplasm do not develop anthers. There are
other types of cytoplasmic defects in corn. I'm seeking feedback about how they manifest.

Other: I'd welcome feedback about how to identify male sterility in other crops.